Long time ago, I saw someone use a software (on Windows) that was specifically created to annotate pictures. It made it simple to add arrows, boxes, circles in "outstanding" colors to the image. Unfortunatly, I don't remember what program that was.

Now, I have to document a GUI and I'd like to use this software in order to annotate screenshots of the software so that I can show the order of flow and dependencies between various aspects of the GUI.

As it currently stands, this question is not a good fit for our Q&A format. We expect answers to be supported by facts, references, or expertise, but this question will likely solicit debate, arguments, polling, or extended discussion. If you feel that this question can be improved and possibly reopened, visit the help center for guidance.
If this question can be reworded to fit the rules in the help center, please edit the question.

As an aside: for true documentation I often find it easier to annotate using the word processor. That way, the screen captures are not changed, and typos or changes can easily be incorporated into the document at a later time. (Remember, on Windows Alt-PrtScr gets you a capture of just the active window; you might want to ensure doing that on a white background because that colour is visible around the rounded corners.) If you're annotating the screen captures directly, then consider using something that allows for removing the annotations at a later time.
–
ArjanJan 21 '10 at 12:44

Thanks everybody for their suggestions. I believe the other suggestions would have equally merited an "accpeted answer", but Greenshot was actually the app I was after.
–
René NyffeneggerJan 21 '10 at 15:08

Second Screenpresso recommendation -- switched from Jing for less resources consumption and less mouse clicks to take a screenshot. Launch it with "-beta" command line argument to get better clipboard handling.

We develop an application, ScreenSteps that is specifcally designed for this type of work. It is different from most screen capture applications in that it doesn't just let you capture and annotate images. It also creates a step by step document for you. Makes the whole authoring process very fast. You can export the results to PDF, HTML, Word or publish to many blogs/wikis. It's available for both Mac and PC.

Windows 7 includes a great utility for this purpose called "Problem Steps Recorder". It allows you to highlight certain parts of a screenshot and attach comments to them, and then exports them to a zipped MHT file.

I used FastStone capture which was free but is now a shareware, and also Fireshot add-on for firefox, to take screenshots of my web apps and annotate it. Note that there's also a "Pro" version and a standalone software.